This investigation will determine the function of a domain of the myosin heavy chain (MHC) that spans from the base of the head to the region where the light chains bind. This domain is likely important in the conformational change that causes the power stroke which results in muscle contraction. It also contains known mutation sites that cause familial hypertrophic cardiomyopathy (FHC), a leading cause of sudden death in young adults. The investigation will be accomplished by expressing a myosin heavy chain transgene in a Drosophila myosin-null mutant. The transgene will encode normal adult MHC isoforms, but will encode the embryonic form of the domain under study. This should result in quantitative changes in mechanical function because the contractile properties of adult fibers differ dramatically from embryonic muscles. The transgenic flies will be assayed for jump and flight ability. The indirect flight muscle (IFM) and tergal depressor of the trochanter muscle (TDT), a jump muscle, will be examined by electron and light microscopy for ultrastructure integrity. Myosin isolated from the animals will be assayed for ATPase activity and nucleotide binding. The ability to produce force and move actin filaments will be assessed by in vitro motility/force generation assays. Maximum velocity of shortening (Vmax) and force production will be measured on isolated IFM and TDT muscle fibers. The results of these assays will reveal the influence of this MHC region on isolated myosin properties and intact muscle function. Future site-directed mutant studies can then determine which amino acids are critical for this domain's function, and may provide insight into why some of these mutations lead to FHC and skeletal muscle central core disease.